Recent advances in paper drying research have identified a phase-change process that offers significant improvements in drying rates and sheet solids contents. A fundamental study focusing on the hypothesized boiling process within the sheet required the development of an apparatus to perform boiling experiments in a bed of ceramic fibers. The apparatus is composed of a boiling cell, a heat supply system, a digital data acquisition system, and associated process control systems for control of heater surface temperature and cell pressure. Boiling curves measured with this system indicate that heat transfer is governed largely by the physical characteristics of the fiber bed. This paper presents a detailed explanation of the boiling apparatus.

Boiling phenomena are investigated in the presence of porous media composed of ceramic fibers ranging in diameter from 3.0 to 18.5 im for system pressures ranging from 0.18 to 0.28 MPa. Bed porosities range from 0.935 to 0.95, average pore diameters range from 30 to 250 ;m, and permeabilities range from 2 x 10-11 to 6 x 10-9 m 2. For surface temperatures up to 400°C, measured boiling curves for water in the different fiber beds show maximum heat fluxes on the order of 25 W/cm 2. The boiling curves exhibit two distinct regimes-a nucleate-type regime and a constant heat flux regime. In the nucleate-type regime, heat flux is directly related to wall superheat, and the slope is directly related to the average pore diameter of the bed. In the constant flux regime, heat flux is independent of wall superheat, as heat transfer is governed by the rate at which the fiber bed supplies water to the heater surface. The transition region can exhibit a peak heat flux and associated rapid escalation in surface temperature similar to burnout in classical pool boiling. Correlations based on dimensional analysis are presented for both regimes. Data in the constant flux regime are fitted to ± 35%, but data in the nucleate-type regime are only fitted to ± 175%.